The environmental and economic benefits of phosphor-converted white-light-emitting diodes (pc-WLEDs) have been increasingly appreciated in recent years. However, a significant challenge in this field pertains to a phenomenon known as thermal quenching, which takes place inside phosphors and leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation. The development of new, more thermally stable phosphors depends on a better understanding of the mechanisms underpinning thermal quenching in phosphors. Here we review the current understanding of thermal quenching mechanisms in Ce3+-doped garnet phosphors, which are widely considered one of the most important families of phosphors for application in pc-WLEDs. In particular, we highlight key structural and dynamical properties, such as the coordination environment of the Ce3+ ions, phonons and local vibrational modes, and structural and chemical defects, which are shown to correlate with phosphor performance. We also discuss the perspectives for future studies in this field in hopes of accelerating the development of new efficient phosphors featuring suppressed thermal quenching of luminescence.

This chapter addresses the development of inorganic phosphor materials capable of converting the near UV or blue radiation emitted by a light emitting diode to visible radiation that can be suitably combined to yield white light. These materials are at the core of the new generation of solid-state lighting devices that are emerging as a crucial clean and energy saving technology. The chapter introduces the problem of white light generation using inorganic phosphors and the structureproperty relationships in the broad class of phosphor materials, normally containing lanthanide or transition metal ions as dopants. Radiative and non-radiative relaxation mechanisms are briefly described. Phosphors emitting light of different colors (yellow, blue, green, and red) are described and reviewed, classifying them in different chemical families of the host (silicates, phosphates, aluminates, borates, and non-oxide hosts). This research field has grown rapidly and is still growing, but the discovery of new phosphor materials with optimized properties (in terms of emission efficiency, chemical and thermal stability, color, purity, and cost of fabrication) would still be of the utmost importance.

Citation: Y Lin et al. "Understanding the interactions between vibrational modes and excited state relaxation in Y3-xCexAl5O12: design principles for phosphors based on 5d-4f transitions.Abstract The oxide garnet Y3Al5O12 (YAG), when a few percent of the activator ions Ce 3+ substitutes for Y 3+ , is a luminescent material widely used in phosphor-converted white 2 lighting. However, fundamental questions surrounding the defect chemistry and luminescent performance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce 3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y3−xCexAl5O12. Increasing the Ce concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-fi splitting due to a larger tetragonal distortion of the CeO8 moieties. Decreasing the Ce 3+ concentration or co-substitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce 3+ doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO8 moieties, and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence.

We report results of the luminescence properties of the three garnet type phosphors Ce3+-doped Ca3Sc2Si3O12 (CSSO:Ce3+), Sr3Y2Ge3O12 (SYG:Ce3+) and Y3Al5O12 (YAG:Ce3+), investigated using optical spectroscopy techniques and vacuum referred binding energy (VRBE) diagram analysis.

Ratio of the radiative and nonradiative transition rates of YAG:Ce3+ phosphor, affected by mode-selective vibrational excitation with an IR laser.

The β−α (order−disorder) transition in the silanides ASiH 3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (C p ), solid-state 2 H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH 3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH 3 − moieties. At temperatures below 200 K ASiH 3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH 3 and KSiH 3 , respectively. The transition is accompanied by a large molar volume increase of about 14%. The C p (T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH 3 − rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH 3 , the SiH 3 − anions undergo rotational diffusion with average relaxation times of 0.2−0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20−40 K). 2 H NMR revealed that the α-form can coexist, presumably as 2−4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from C p measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH 3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH 3 − ions are only weakly coordinated in an environment of A + cations. The orientational flexibility of SiH 3 − can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.

The molecular orientation distribution of polymeric fibers influences physical properties. We present a novel method of analyzing polarized Raman experiments to determine molecular orientation, which is based on exchanging the Legendre polynomial approach with a wrapped Lorentzian function, as determined from a prescreening of X-ray scattering patterns. This method removes the need for performing right angle scattering experiments while avoiding common approximations. The molecular orientation of regenerated cellulose fibers, using the presented method, is shown to correlate well with X-ray scattering and an analogous experiment using solid-state NMR spectroscopy. Challenges of quantitatively measuring molecular anisotropy occur with semi-crystalline, partially modified, or composite materials. As such, a plethora of techniques, each with a unique chemical selectivity, is paramount for material characterization.

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